1. Field of the Invention
The invention is related to a method and apparatus for generating a low-temperature, atmospheric pressure plasma, and its use for surface treatment and for coating substrates. In particular, the invention is related to a method and apparatus of generating a low-temperature, atmospheric pressure plasma, wherein the plasma produces a linear beam of reactive gas species that is well suited for processing substrate surfaces at high speeds. The invention is further related to specialized products that are made using the method and apparatus for generating the low temperature, atmospheric pressure plasma.
2. Description of the Related Art
Plasmas are employed in materials manufacturing for a diverse range of processes, including surface activation, etching, cleaning, sterilization, decontamination, and thin film deposition. Plasmas operate either at low pressure, <5 Torr, or at atmospheric pressure, ˜760 Torr (see for example, Lieberman and Lichtenberg, “Principles of Plasma Discharges and Materials Processing,” John Wiley & Sons, Inc., New York, 1994; Chen, “Introduction to Plasma Physics and Controlled Fusion,” Plenum Press, New York, 1984; and Roth, “Industrial Plasma Engineering: Vol. I, Principles” Institute of Physics Publishing, Philadelphia, Pa., 1995). The low-pressure devices are operated in a batch mode, and find wide application in semiconductor fabrication. By contrast, atmospheric pressure plasmas may be operated in a continuous mode on an assembly line, and are more common in automotive, aerospace, and specialty materials industries.
Low-temperature, atmospheric pressure plasmas are weakly ionized discharges, such that only a small fraction of the gas molecules become ionized (see Sch{umlaut over (t)}ze, et al., “The Atmospheric-Pressure Plasma Jet: A Review and Comparison to other Plasma Sources,” IEEE Transactions in Plasma Science, vol. 26, page 1685 (1998)). These systems are not at equilibrium, because the temperature of the free electrons is several orders of magnitude higher that the temperature of the neutral species. Several types of non-equilibrium, atmospheric pressure plasmas have been developed over the years. These include coronas, dielectric barrier discharges, micro hollow cathode discharges, and radio frequency powered, capacitive discharges.
A corona is an electrical discharge where ionization takes place in a region of high electric field. The most common type of corona is the point-to-plane design, where one of the electrodes is a narrow wire or a metal tip and the other electrode is planar (see Goldman and Goldman, “Corona Discharges” Gaseous Electronics, vol. 1, (Eds: Hirsh and Oakam), Academic Press, New York, 1978). Power, at frequencies ranging from 50 Hz to 20 kHz, is supplied to the pointed electrode, creating a high electric field that promotes breakdown of the gas in the vicinity of the electrode. A localized, luminous discharge is created around the tip of the powered electrode. Since the plasma density falls rapidly away from the sharp tip, one must pass the substrate very close to the electrode for the substrate to be processed at a suitable rate. Therefore, this device is for the most part restricted to treating plastic film or fabric that is continuously passed through the plasma in a roll-to-roll format.
Dielectric barrier discharges, also known as “silent” discharges, operate with two metal electrodes, in which at least one is coated with a dielectric material. The metal electrodes are separated by a uniform gap, and are powered by DC or AC at frequencies up to 20 kHz. In most cases, dielectric barrier discharges operate in a “filamentary” or “microdischarge” mode, where the plasma exhibits short-lived micro arcs that are randomly distributed in space and time (see Eliasson and Kogelschatz, IEEE Transactions in Plasma Science, vol. 19, page 1063, 1991). A uniform, diffuse glow mode can be obtained in a dielectric barrier discharge if an inert gas such as helium, argon, or nitrogen is used as a diluent. The electron density in these plasmas varies over a wide range depending on whether the gas is sampled inside or outside a streamer. Nevertheless, the average electron density is low, ˜109 cm−3, which means that like a corona, one must insert the substrate into the plasma between the electrodes to obtain a suitable surface treatment rate. Dielectric barrier discharges are primarily employed in the surface activation of plastic film.
Microhollow cathode discharges are direct-current glow discharges sustained between two parallel metal electrodes with a center opening of 0.1 mm in diameter in either the cathode, or the cathode and the anode (see Stark and Schoenbach, Applied Physics Letters, vol. 74, page 3770, 1999; and Bardos and Barankova, Surface Coating Technologies, vol. 133-134, page 522, 2000). The electrodes are separated by a gap of 0.2 to 0.4 mm, which is often filled with a dielectric material. Gas, such as argon, xenon or air is passed through the hole where it is ionized by application of DC, or in a few cases, RF power. The plasma density is highest inside the hole at 1014 cm−3, and quickly decreases in density outside of this region. Hollow cathode discharges are mostly used as light sources and processing materials with these devices has been limited.
A nonequilibrium, atmospheric pressure discharge may be produced by flowing gas between two closely spaced metal electrodes that are driven with high-frequency power (see Koinuma et al., U.S. Pat. No. 5,198,724; Li et al., U.S. Pat. Nos. 5,977,715 and 6,730,238; and Selwyn, U.S. Pat. No. 5,961,772). These plasmas have been used to process materials placed a short distance downstream of the electrodes. The disadvantage of these designs is that the plasma beam is produced as a small spot. In addition, the concentration of reactive species generated in these plasmas is relatively low. These two facts taken together, mean that the rate of processing objects with a reasonable amount of surface area is too slow to be of practical interest. For example, Jeong et al. (Journal of Vacuum Science and Technology A, vol. 17, page 2581, 1999) showed that a 4 mm spot on a Kapton sheet is etched at a maximum speed of 8.0 microns per minute. To etch through a 25-micron sheet one-foot square would take about 16 days!
In patent application publication US 2002/0129902 A1, “Low-Temperature Compatible Wide-Pressure-Range Plasma Flow Device,” dated Sep. 17, 2002, Babayan and Hicks describe an apparatus that comprises a housing with two perforated metal electrodes. Gas flows through the electrodes and is partially ionized by applying radio frequency power to one of the electrodes at 13.56 MHz. Radicals produced in the plasma flow out of the device and may be used to treat substrates placed a short distance downstream. It was observed that the etch rate of photoresist with an oxygen and helium plasma at 760 Torr was between 0.4 and 1.5 microns per minute over a circular area 30 mm in diameter. Note that the plasma device was placed close to the substrate, i.e., only 3 mm away, which is somewhat of a disadvantage for treating 3-dimensional objects.
Thus, there is a need for a low temperature, atmospheric pressure plasma that generates a linear beam of reactive gas over a wide range of distances such that the plasma can be used to rapidly treat both flat and 3-dimensional substrates of any size or shape.